Signal inverting circuits have many applications and find wide usage in logic networks to perform various functions, in vital systems, for example, in railroad mass and/or rapid transit operations. In the past, the necessary logic functions were accomplished by electrical and electronic devices and networks, such as, electromagnetic relays and transistor circuits. While electromagnetic relays operated successfully in previously designed logic systems, their usage was accompanied by several shortcomings, such as, bulkiness in size and weight, slowness of response, namely, long pick up and drop out times, sensitivity to shock and vibrations and susceptibility of contact bounce and wear. Similarly, in transistor logic systems some difficulty has been encountered in regard to interfacing and isolation, and in many transistor logic arrangements ancillary self-checking circuits were generally utilized in vital applications in order to ensure that the system was operating properly and that no critical failure was present. It has been found an opto-electronic inverter employing optical coupling devices perform very satisfactorily and provide additional advantages over those achieved by prior art inverters. For example, optical couplers exhibit low capacitance from input to output and no feedback occurs from the output to the input. Other attributes possessed by optical couplers are fast transfer speed, small size, high immunity to shock and vibrations, wide range of frequency response and long lived. In addition, optical couplers have a wide range of current transfer ratios which may vary from fractions of a percent to hundreds of percent. Thus, the characteristics and versatility of optical couplers make them ideally suited for direct use with logic circuits and for interfacing power with logic or vice versa and logic system with logic system due to their extremely good isolation qualities. Further, it will be appreciated that in any vital system it is of paramount importance to exercise extreme care in designing and assembling the various networks and circuits of the system in order to preclude possible injury to individuals and prevent costly damage to equipment in case a critical component or circuit fails. That is, it is a fundamental authoritative requisition that under no circumstance will any conceivable failure cause or be capable of causing a true or valid output condition. This stringent requirement for vitality is essential in order to provide the highest degree of safety to individuals as well as apparatus. In order to achieve such a vital operation, the present inverting circuit has been specifically designed in accordance with the fail-safe principles approved by the Association of American Railroads (AAR). Thus, a critical circuit or component failure, such as, an open or short circuited malfunction must not be capable of erroneously simulating a valid output condition.